Microstrip latched ferrite phase shifter wherein latching pulses pass through ground plane

ABSTRACT

A latching ferrite phase shifter for microstrip transmission lines compatible with integrated circuits and incorporating two ferrite materials, one of which is sandwiched between the transmission line and a ground plane and the other of which is on the side of the ground plane opposite the transmission line. The ferrite material between the transmission line and the ground plane is designed to control the amount of phase shift produced by the phase shifter; while the other ferrite material on the side of the ground plane opposite the transmission line is isolated from microwave frequencies but forms a magnetic circuit with the first and has a coercive force and saturation magnetization much greater than that of the first to hold it in a magnetized state above its remanent point.

United States Patent [72] Inventors DauielC.Budt Hanover, Md.;Aleksander l. Braginski. Pittsburgh, Pa. [2]] Appl. No. 26,199 [22]Filed Apr. 7, 1970 [45] Patented Aug. [0,1971 [73] Assignee WestinghouseElectric Corporation Pittsburgh, Pa.

[54] MICROSTRIP LATCHED FERRITE PHASE SHIFT ER WHEREIN LATCHING PULSESPASS THROUGH GROUND PLANE 9 Claims, 3 Drawing Figs.

[52} U.S. Cl 333/31 R, 333/24.l [Sl] Int.Cl H0lp 1/18, H01 p H32 [50]Field of Search 333/24.l, 24.2, l.l

DE-LATCHING CURRENT DIRECTION [56] References Cited UNITED STATESPATENTS 3,447,l43 5/1969 Hair et al. .r 333/24.1X

Primary Examiner- Herman Karl Saalbach Assistant Examiner Paul L.Gensler Attorneys-F. H. Henson, E. P. Klipfel and D Schron ABSTRACT: Alatching ferrite phase shifter for microstrip transmission linescompatible with integrated circuits and incorporating two ferritematerials, one of which is sandwiched between the transmission line anda ground plane and the other of which is on the side of the ground planeopposite the transmission line. The ferrite material between thetransmission line and the ground plane is designed to control the amountof phase shift produced by the phase shifter; while the other ferritematerial on the side of the ground plane opposite the transmission lineis isolated from microwave frequencies but forms a magnetic circuit withthe first and has a coercive force and saturation magnetization muchgreater than that of the first to hold it in a magnetized state aboveits remanent point.

PATENTEU AUG! 012m (\LATCHING FIELD W N w 66 T T ET HEC LR R RE C E summmm DCD LCD MICROSTRIP LATCHED F ERRITE PHASE SHIFTER WHEREIN LATCI-IINGPULSES PASS THROUGH GROUND PLANE CROSS-REFERENCES TO RELATEDAPPLICATIONS Application Ser. No. 821,344, filed May 2, 1969 andapplication Ser. No. 11,453, filed Feb. 16, 1970, both applicationsbeing assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION As is known, most prior art ferrite phaseshifters have utilized flat slabs of ferrite material together with abiasing directcurrent field. For switching, an electromagnet wassometimes used; but this resulted in relatively slower switching speeds.The development of digital latching phase shifters has eliminated theneed for holding fields and has made possible submicrosecond switchingspeeds. In these designs, ferritemagnetic toroids of various lengths areplaced in a wave guide and latching wires passed through the centers ofthe toroids. By applying a current pulse of appropriate magnitude to theends of the latching wire, the ferrite material can be driven close tosaturation to effect a desired phase shift and will remain in a remanentmagnetized state until a pulse of the opposite polarity is appliedacross the ends of the latching wire to drive the ferrite material outof its magnetized state. The toroid configuration is used since, amongother reasons, it forms a complete magnetic circuit which facilitatesretention of remanent magnetization once the pulse is removed.

With the availability of microwave transistors and other semiconductordevices usable at microwave frequencies, the microstrip transmissionline has found wide application because of its compatibility with thefabrication and installation of passive components and active devices onthe same substrate with the transmission line. Essentially, a microstriptransmission line is similar in operation to acoaxial TEM mode wavetransmission line and consists of a strip of conductive material,corresponding to the center conductor of a coaxial transmission line,deposited on one side of a dielectric or semiconductive substrate, e.g.,by photoresist techniques. The opposite side of the substrate is coveredwith a layer of conductive material comprising a ground plane andcorresponding to the outer cylindrical conductor of a coaxialtransmission line. With this configuration, and assuming that a sourceof signal wave energy is applied across the strip and ground plane onopposite sides of the substrate, an electromagnetic field pattern isestablished between the two.

In order to provide a latching phase shifter for such microstriptransmission lines, it is necessary to dispose the ferrite material inthe space between the ground plane and the microstrip conductor itself,meaning that the thickness of the ferrite material has to be close tothat of the integrated circuit substrate, a typical example being about5 to 50 mils in the case of X-band and decreasing with increasingfrequency. This poses a problem for microstrip latching ferrite phaseshifters for two reasons. The first is that while the ferrite materialcan be driven into saturation by an appropriately applied pulse,difficulty is encountered in maintaining the flux density in the regionof saturation. This is due to the fact that the ferrite film does notform a complete magnetic circuit as is the case, for example, withtoroids used in conventional wave guides, and the film thickness tosurface area ratio is high enough to create a demagnetizing field whichreduces the effective remanent state much below the true materialremanence. The second reason is that the film material must be optimizedfor microwave performance and to reduce insertion losses. When thisrequirement is satisfied, however, the material has an inadequateremanent magnetization value much below that of the saturated state.

In copending application Ser. No. 11,453, filed Feb. 16, 1970, amicrostrip latching phase shifter is disclosed wherein many of theproblems encountered in maintaining a ferrite film in a magnetizedcondition are obviated by providing a second magnetic material of highercoercive force in a magnetic circuit with the active ferrite filmitself, this second material acting to insure that the active ferritefilm will attain a desired degree of magnetization. This is accomplishedin accordance with the teachings of that application by providing twoferrite films, one of which is in the path of the electromagnetic wavepassing along the microstrip circuit and is designed to control theamount of phase shift produced by the phase shifter. The other of theferrite materials is deposited over the microstrip conductor andexercises a magnetostatic interaction with the first, or forms acomplete magnetic circuit with the first, and has a coercive forcegreater than that of the first to hold the first in the magnetizedstate. Since the coercive force of the second ferrite film above themicrostrip conductor is greater than that of the active ferrite filmitself, the active film can be held at a higher state of magnetizationthan would otherwise be the case, thereby increasing the phase shiftattainable.

While the device shown in the aforesaid copending application isentirely satisfactory for its intended purpose, its performance cansuffer from the fact that the electric field between the microstripconductor in the ground plane on the other side of the active ferritefilm will also intersect the upper ferrite film of higher coerciveforce. This means that the characteristics of the upper film, and inparticular its saturation magnetization, must be optimized for microwavefrequencies. For each operating frequency, there is a maximum value ofsaturation which can be used without encountering excessive insertionlosses. This means, in effect, that the saturation magnetization of bothfilms must be essentially the same.

SUMMARY OF THE INVENTION In accordance with the invention, a microstriplatching ferrite phase shifter is provided incorporating two ferritematerials, one of which is in the path of an electromagnetic wavepassing along the microstrip and the other of which acts as a keeper tohold the first in a desired magnetized state, but wherein the secondfilm is not in the path of the electromag netic wave and need not be amicrowave ferrite material. On the contrary, it can be optimized withrespect to its magnetization characteristics only in particular, itssaturation magnetization can be much greater than that of the activeferrite film.

The foregoing is accomplished by providing a second keeper" magneticfilm beneath the ground plane of the latching circuit instead of on topof the microstrip. A separate latching wire for the phase shifter is noweliminated since the latching pulses can pass through the ground planeitself. After pulsing the ground plane, the microstrip circuit activeferrite is driven above the remanent state by the base ferrite ofappropriately chosen magnetization characteristics. Preferably, theferrites used are nickel ferrites and the ground plane is formed fromgold to reduce insertion losses over those which would be encounteredwith other ferrite materials using platinum as the ground planematerial.

The above and other objects and features of the invention will becomeapparent from the following detailed description taken in connectionwith the accompanying drawings which form a part of this specificationand in which:

FIG. 1 is a partially broken away perspective view of one embodiment ofthe invention;

FIG. 2 is a partially broken away perspective view of another embodimentof the invention; and

FIG. 3 illustrates the hysteresis curves of two typical superimposedferrite films utilized in the invention shown in FIGS. 1 and 2.

With reference now to the drawings, and particularly to FIG. 1, theembodiment of the invention shown includes an active ferrite film 10having a ground plane 12 deposited on its lower surface. Beneath theground plane 12 is a second magnetic layer 14 which, as will be seen,acts as a keeper" to maintain the upper active ferrite film 10 in adesired magnetized state. a

Deposited on the upper surface of the active ferrite film is a microwavetransmission line or microstrip 16 formed as a meander line. It includesa number of convolutions having long, parallel legs 16a and short,parallel legs 16b at right angles to the legs 16a. The legs 16a definethe primary direction of propagation of microwave energy through themicrostrip transmission line 16. Each of the ferrite films 1'0 and 14,the ground plane 12 and the microstrip transmission line 16 may beformed by suitable deposition techniques.

As was mentioned above, the electromagnetic wave signal propagatesbetween the ground plane 12 and the meander line 16 in the direction ofthe legs 16a. The magnetization of the ferrite film 10, therefore,controls the phase shift in the microstrip. Since the lower "keeper"ferrite layer 14 is separated from the microstrip meander line 16 by theground plane 12, its characteristics, and particularly its saturationmagnetization need not be optimized for microwave frequencies.

The magnetization of the films 10 and 14 is controlled by a currentpulse generator 20 connected to the ground plane 12 as shown in FIG. 1through switches 22 and 24. When both switches 22 and 24 are in thepositions shown in FIG. 1, the current pulse will pass through theground plane 12 in the direction of arrow 26, thereby producing alatching field H, which is parallel to the direction of signalpropagation along the meander line 16. Note that the four corners of theground plane are cut away as at 28 so that the upper ferrite film 10contacts the lower film 14 in these areas to provide a complete magneticcircuit. The magnetic field I'l being parallel to the direction ofsignal propagation, provides for maximum phase shift.

Alternatively, when the positions of the switches 22 and 24 are reversedwith respect to those shown in FIG. 1, a current pulse will flow throughthe ground plane 12 in the direction of arrow 30, thereby producing amagnetic field l-I which is at right'angles to the direction of signalwave propagation. Under these circumstances, the phase shift is aminimum. Thus, by reversing the positions of switches 22 and 24, thephase shift effected by the phase shifter can be switched from a maximumto a minimum.

The formula for differential phase shift per unit length, AB, for aferrite-loaded microstrip is:

where B =21rf/ V,,= phase constant for unmagnetized ferrite filledmicrostrip;

V,,= the microstrip phase velocity c/ e,= relative dielectric constantof the ferrite;

m,,,=(2.8 MHz./gauss) X41rM; and

41rM= magnetization in gauss. Phase shift per unit length is a verysteep function of the factor (Om/(1). In order to maximize the phaseshift per unit length, it is necessary to operate at as high a wm/wvalue as possible with the side condition that w,,,,, corresponding to41rM,,, the saturation magnetization, is not too close to the operatingfrequency m. In a conventional latched phase shifter, a) will always beless than m corresponding to the remanent magnetization. The remanentmagnetization M,, is always substantially less than M,,,,, often by afactor of about 1.5, with a corresponding drastic increase in phaseshift per unit length. When the phase shift per unit length isdecreased, the length for 360 phase shift is increased, with acorresponding increase in insertion loss.

In the present invention, the factor tum/w can be maximized by virtue ofthe fact that the keeper" ferrite layer 14 is beneath the ground plane12 and, therefore, is out of the path of the microwave energy passingalong the meander line 16. As a result, its saturation magnetization canbe greatly increased. This is shown in FIG. 3 wherein hysteresis curvesfor the ferrite layers 10 and 14 are shown. The hysteresis curve forferrite 14 has a much larger coercive force H than that,

H for ferrite 10. At the same time, however, it has amuch highersaturation magnetization M than that of the ferrite 10, M Likewise, theremanent magnetization M of ferrite 14 is much higher than for that, Mof ferrite 10. The result is that by driving the ferrite 14 to point 32on its hysteresis curve, the microwave ferrite 10 can be driven to point34, far above its remanent magnetization M As was explained above, thisincreases the value of com/w and reduces insertion losses.

For the case given in FIG. 3, it is assumed that the cross-sectionalareas of the layers 10 and 14, perpendicular to the direction of fluxpropagation, is the same. Since the flux density in the two layers isthe same, the magnetization, M, in both layers must be the same (i.e.,points 32 and 34 must have the same ordinate. The demagnetizing field inlayer 14, H5, is equal to the demagnetizing field, H in layer 10. Ofcourse, if the thickness of layer 14 is greater than that of layer 10,then point 34 can be higher than point 32 since the magnetization oflayer 10 can be greater than that of layer 14.

Calculations indicate that by using the system of the invention, it ispossible to achieve a 37 percent improvement in figure of merit; wherefigure of merit is defined as phase shift divided by insertion loss.This 37 percent improvement in figure of merit assumes the use of nickelferrites and a gold latching circuit. The importance of using nickelferrite and a gold latching circuit arises from the fact that thelatching circuit also plays the role of a microstrip ground plane and,therefore affects the insertion loss. When magnesium-manganese ferritesare employed, platinum has to be used as a ground plane because of thehigh reaction temperature which is above the melting point of gold. Theresistivity of platinum, however, is about 6.1 times that of copper;whereas the resistivity of gold is only about 1.5 times that of copper.Consequently, the nickel ferrite-gold ground plane arrangement is highlydesired over magnesium-manganese-platinum systems in order to achievelow insertion loss phase shifters; however the latter system can stillbe used (i.e., Mg-Mn-Pt) with improvement over prior art devices.

With reference now to FIG. 2, another embodiment of the invention isshown wherein elements corresponding to those as shown in FIG. 1 areidentified by like'reference numerals. In this case, however, thecorners of the ground plane I2 are not cut away. Rather, slots ornotches 36 are provided at the four edges of the ground plane 12 toprovide complete microwave circuits forlatching field H, and thedelatching field H The connections to the pulse generator are not shownin FIG. 2; however it will be appreciated that they are essentially thesame as that shown in FIG. 1. Note that the meander line 16 is bent soas not to lie over any of the notches 36.

Although the invention has been shown in connection with certainspecific embodiments, it will be readily apparent to those skilled inthe art that various changes in form and arrangement of parts may bemade to suit requirements without departing from the spirit and scope ofthe invention.

We claim as our invention:

1. A latching ferrite phase shifter comprising a layer of ferritematerial, a metallic microstrip conductor deposited on one side of saidlayer of ferrite material, a ground plane deposited on the other side ofsaid layer of ferrite material, a second layer of ferrite materialdeposited on the side of said ground plane opposite said first-mentionedlayer, means for connecting a source of latching pulses to two oppositesides of said ground plane, means for connecting a source of delatchingpulses to the other two opposite sides of said ground plane, andconnections between at least portions of said ferrite layers to providecomplete magnetic circuits for the magnetic fields produced by saidlatching and delatching pulses respectively.

2. The latching ferrite phase shifter of claim 1 wherein saidconnections between portions of said ferrite layers are provided bycutaway portions at the four corners of said ground plane.

3. The latching ferrite phase shifter of claim 1 wherein the connectionsbetween said portions of the ferrite layers are provided by slots in thefour edges of said ground plane.

4. The latching ferrite phase shifter of claim 1 wherein said ferritelayers comprise nickel ferrite and wherein said ground plane is formedfrom gold.

5. The latching ferrite phase shifter of claim 1 wherein said metallicmicrostrip conductor comprises a meander line.

6. The latching ferrite phase shifter of claim 1 including a pulsegenerator, and switch means for connecting said pulse generatoralternatively to one pair of opposite sides of the ground plane and thenthe other pair of opposite sides.

7. The latching ferrite phase shifter of claim 1 wherein said

1. A latching ferrite phase shifter comprising a layer of ferritematerial, a metallic microstrip conductor deposited on one side of saidlayer of ferrite material, a ground plane deposited on the other side ofsaid layer of ferrite material, a second layer of ferrite materialdeposited on the side of said ground plane opposite said first-mentionedlayer, means for connecting a source of latching pulses to two oppositesides of said ground plane, means for connecting a source of delatchingpulses to the other two opposite sides of said ground plane, andconnections between at least portions of said ferrite layers to providecomplete magnetic circuits for the magnetic fields produced by saidlatching and delatching pulses respectively.
 2. The latching ferritephase shifter of claim 1 wherein said connections between portions ofsaid ferrite layers are provided by cutaway portions at the four cornersof said ground plane.
 3. The latching ferrite phase shifter of claim 1wherein the connections between said portions of the ferrite layers areprovided by slots in the four edges of said ground plane.
 4. Thelatching ferrite phase shifter of claim 1 wherein said ferrite layerscomprise nickel ferrite and wherein said ground plane is formed fromgold.
 5. The latching ferrite phase shifter of claim 1 wherein saidmetallic microstrip conductor comprises a meander line.
 6. The latchingferrite phase shifter of claim 1 including a pulse generator, and switchmeans for connecting said pulse generator alternatively to one pair ofopposite sides of the ground plane and then the other pair of oppositesides.
 7. The latching ferrite phase shifter of claim 1 wherein saidsecond layer of ferrite material has a higher saturation magnetizationthan that of said first layer.
 8. The latching ferrite phase shifter ofclaim 1 wherein said second layer of ferrite material has a highercoercive force and higher saturation magnetization than saidfirst-mentioned layer of ferrite material.
 9. The latching ferrite phaseshifter of claim 1 wherein said second layer of ferrite material has ahigher coercive force and higher saturation magnetization than saidfirst-mentioned layer and wherein said first-mentioned layer ismagnetized to a point above its remanent magnetization by a latchingpulse.